Superposed drilling devices comprise an external pipe string and an internal pipe string extending coaxially thereto which, in common, are driven into rock or ground to produce a borehole whose wall is supported by the external pipe string. To backflush the bore material, a scavenger medium such as air or water is conducted through the internal pipe string and to the borehole bottom. The scavenger supplied to the internal pipe string is introduced through the hammer drill mounted at the rear end of the rotary drive. A scavenging pipe secured to the hammer drill housing traverse's a bore of the piston of the hammer drill to project into a scavenging bore of the shank receiving the blows of the hammer piston and being rotatingly driven by the rotary drive. The shank is integrally connected to the internal pipe string, and its scavenging bore communicates with the bore of the internal bore string.
In the known superposed drilling devices in which the scavenger is supplied through the hammer drill, the seal between scavenging pipe and scavenging bore is provided in an annular groove of the scavenging bore. The scavenging pipe rubs along said seal which is subjected to a high wear due to the continuous linear movements between shank and scavenging pipe, on the one hand, and due to the considerable pressure fluctuations and high pressure peaks to which the seal is exposed. While the piston of the hammer drill strikes the shank, the shank together with the pipe string connected thereto moves in axial direction, however, the column of scavenger medium in the pipe string is not accelerated. With each blow, the static pressure of the scavenger is superposed by a dynamic pressure which may be very high. Due to said high pressures, it is necessary to maintain a very close tolerance between the external diameter of the scavenging pipe and the diameter of the scavenging bore containing the moving scavenging pipe. In view of the impact stress for the shank, the material strains are considerable with the result, that the scavenging pipe is exposed to a high risk of break. As a matter of fact, a break of the scavenging pipe is very frequently responsible for the failure of the drilling device. Since the seal is provided in an annular groove of the shank receiving the blows of the hammer drill, the shank is weakened near the striking zone. Breaks of the shank are not unusual either accordingly.
Due to the foregoing difficulties, the scavenger supply through the hammer drill and through the shank were performed in the past by only allowing a maximum inner diameter of about 11 mm and a maximum pressure of the scavenger of about 7 bar, because, otherwise, there has been the risk of destroying the seal, the scavenging pipe or the shank. In view of the small diameter of the scavenger bore and of the low pressure, the amount of scavenger supplied through the inner pipe string has been insufficient for effective flushing. Therefore, in case of larger borehole diameters, additional scavenger has been introduced into the external pipe string through a scavenging ring which is retained relative to the rotating external pipe string.
The scavenger amounts supplied through the internal pipe string and the external pipe string are flushed back long the outside of the external pipe string thus entailing the risk of a wash-out of the borehole.
In case of superposed drilling, it is of interest to produce boreholes as big as possible at a high driving speed. A high scavenger flow rate is required to this effect. Due to the problems of tightness and stability, the scavenger channel of the known superposed drilling device may not be dimensioned big enough to guide a sufficiently high amount of scavenger only through the internal pipe string